1. Field of the Invention
The present invention relates to an alloy-free press contact type semiconductor device which is employed for power use or the like, and more particularly, it relates to the shape of an insulator which is provided on the outer peripheral part of a semiconductor substrate.
2. Description of the Background Art
FIG. 10 is a sectional view showing a conventional alloy-free press contact type semiconductor device, which is described in Japanese Patent Laying-Open Gazette No. 1-215028 (1989), for example. Referring to the figure, numeral 1 denotes a cathode copper block, numeral 2 denotes an anode copper block, numeral 3 denotes a casing consisting of an insulator such as ceramic, numeral 4 denotes an anode heat compensator, numeral 6 denotes a semiconductor substrate, numeral 7 denotes an insulator consisting of silicone rubber or the like, numeral 8 denotes a spring, numeral 9 denotes a gate lead, numeral 10 denotes a gate support rod, and numeral 11 denotes an insulating varnish. The semiconductor substrate 6 is provided in its interior with a P-N junction, and its outer peripheral edge is insulated by the insulating varnish 11, while high voltage resistance is attained by the insulator 7.
The insulator 7 is adapted to increase a creeping distance for attaining high voltage resistance, while positioning the cathode and anode heat compensators 5 and 4 with respect to the semiconductor substrate 6.
After the respective parts are assembled as shown in FIG. 10, the cathode and anode copper blocks 1 and 2 are pressurized to bring the respective parts into press contact with each other, for attaining prescribed electrical properties.
FIGS. 11 and 12 are sectional views showing a portion around the insulator 7 which is formed on the outer periphery of the semiconductor substrate 6 in an enlarged manner. Referring to FIG. 11, an inner peripheral surface 12 of the insulator 7 is formed to be perpendicular to the surface of the semiconductor substrate 6. As shown in FIG. 12, the insulator 7 is cast-molded and hardened/fixed through upper and lower molds 15 and 16, and thereafter released from the molds. In such mold releasing, separation 17 is disadvantageously caused in a bonded surface of the inner periphery. When a tapered part 13 is formed on the inner periphery of the insulator 7 as shown in FIG. 13 in order to solve this problem, the cathode heat compensator 5 or the anode heat compensator 4 goes aground on the inner peripheral edge of the insulator 7, defines a bite portion 18, and comes into half-contact with the semiconductor substrate 6.
While FIG. 10 shows a structure which is provided with a gate at the center, a similar problem also arises in the case of the so-called outer peripheral ring gate structure which is provided with the gate on the outer periphery of the semiconductor substrate 6.
In order to dissipate heat which is generated when the press contact type semiconductor device is driven, on the other hand, it is effective to maximize the diameters of the anode and cathode heat compensators 4 and 5. When a foreign matter 40 is present on the outer periphery of the semiconductor substrate 6 as shown in FIG. 14, however, a short is inevitably caused across a cathode electrode 19 and a gate electrode 20 of the semiconductor substrate 6 through the cathode heat compensator 5, and hence the diameter of the cathode heat compensator 5 cannot be increased. Thus, it is disadvantageously difficult to position the small cathode heat compensator 5 although the large-diameter anode heat compensator 4 can be readily positioned.